I shot a lot with the 5d mk iii and taped pins on a 1.4 ii/400 f5.6 combo, and I was pretty impressed with this combo, with the new firmware I removed the tape and expected not much difference. What I found was the focus speed felt a little bit slower, but, locking on quicker and more accurately. No more jerky hunting.
As far as the image quality goes, you still pay a price, but, when I am further than 6m(20ft) and I am in good light the extender combo is better than cropping, anything closer the bare lens and cropping are better.
Its always hard to decide if I should mount the extender or not, It is capable of BIF but. not as fast. If I knew that I was going to take pictures of stationary birds it would be okay. If I could afford it, I would preorder the 200-400 but that is not an option(My Wife clarified that for me).